The present invention relates to an apparatus, in particular a coordinate measuring machine, for inspecting a measurement object, comprising a workpiece support for supporting the measurement object, comprising a measuring head carrying an optical sensor, wherein the measuring head and the workpiece support are movable relative to one another, wherein the optical sensor has an objective and a camera, which is designed to capture an image of the measurement object through the objective, wherein the objective has a light entrance opening and a light exit opening, wherein the objective has a multitude of lens-element groups which are arranged in the objective between the light entrance opening and the light exit opening one behind another along a longitudinal axis of the objective.
Furthermore, the present invention relates to a method for calibrating an apparatus for inspecting a measurement object.
The use of optical sensors in conjunction with coordinate measuring machines makes it possible in many cases to measure geometrical properties of a measurement object very rapidly. One disadvantage of known coordinate measuring machines comprising optical sensors heretofore has been that the optical sensors are limited to specific measurement tasks and specific workpiece properties. The optical sensors are generally optimized for a specific type of measurement task, for instance with regard to the achievable measurement accuracy or the measurement range. Problems can be posed for example by workpieces which have large height differences parallel to the optical axis of the sensor. In part, different optical and/or tactile sensors are used in order to be able to react flexibly to different measurement requirements, wherein the individual sensors in each case perform only part of the overall measurement task. In general, each individual sensor is optimized towards a specific measurement task. Primarily optical sensors therefore have a respective individual optics which is well suited to a specific purpose of use and is less well suited to other purposes.
In this case, such an optics has to be checked at regular intervals. This serves to detect, and if appropriate compensate for, possible changes in the positioning of the optical elements relative to one another, whether as a result of physical external influences, such as e.g. collisions, or e.g. on account of thermal influences.
If optics having a variable imaging scale are used, the optical elements are typically adjusted, with the aid of actuating cams. In this case, either the lens elements can be coupled by means of a cam disk, or else the position of the optical elements relative to one another can be set by means of separate drives. In general, one relies here on the actuating accuracy of the actuating cams or of the drives. However, these can also change over time.
In order to check e.g. set magnifications or actually the imaging behavior of an optics, at specific test intervals a test body having a known structure in terms of geometry and/or size can be placed below the optics or the objective and a size of the imaged structure on the camera can be measured. In this way, the entire beam path can then be calibrated with regard to magnification, the imaging scale or actually the optical properties of the objective. In principle, it is then also possible to readjust the magnification or the imaging scale if the image deviates from the desired imaging scale by more than a step size of the drive.
Typical calibration objects are the glass carriers that are lacquered, if appropriate, on the rear side. The structure is then applied on the top side or front side. Calibration objects are positioned in the apparatus and then measured.
Calibration methods which can operate with the positioning of a measurement object is mentioned e.g. in the document DE 10 2009 054 703 A1. A coordinate measuring machine is disclosed by the document WO 99/53268 A1, for example.
Since a user has to handle the calibration objects, there is the risk of said calibration objects being soiled or even scratched. Moreover, a user has to implement and typically also accompany the calibration expenditure, which costs work time.
It is an object of the present invention to eliminate the disadvantages mentioned above and to provide improved possibilities for calibration in an apparatus for inspecting a measurement object.